The repair of tendons or ligaments is a challenging and complication prone area of surgery. As one example, the dilemma in flexor tendon repair surgery in the hand is to adequately connect a severed tendon without compromising the functionality of the hand due to surgical intervention and repair techniques.
Tendons can sustain high tensile forces resulting from muscle contraction, yet are flexible enough to bend around bony surfaces and deflect beneath retinacula to change the final direction of muscle pull. Tendons attach muscle to bone and transmit tensile loads from muscle to bone thereby producing joint movement. Ligaments attach bone to bone and can flex to allow natural movement of the bones that they attach, but are strong and inextensible so as to offer suitable resistance to applied forces. Ligaments augment the mechanical stability of the joints. The biomechanical behavior of tendons and ligaments is viscoelastic or rate dependent, that is, their strength and stiffness increase with an increased loading rate. Bundles of collagen fibers embedded in a connecting matrix, known as ground substance, provide the load carrying elements of natural tendons and ligaments. The arrangement of the collagen fibers is nearly parallel in tendons, equipping them to withstand high unidirectional loads.
The less parallel arrangement of the collagen fibers in ligaments allows these structures to sustain predominant tensile stresses in one direction and smaller stresses in other directions. The ground substance in both tendons and ligaments acts generally as a cementing matrix holding the collagen fibers together. The ground substance retains large amounts of water essential to the non-compressive hydraulic function of the moving tissue. Also included in the tendon composition are elastic fibers, tenocytes, small blood vessels and nerves. In general, the cellular material (fibroblasts) occupies about 20% to 38% of the total tissue volume, while the ground substance matrix accounts for the remaining 62% to 80%. About 70% of the ground substance matrix consists of water absorbed in an open polysaccharide matrix.
Two types of tendons exist in the hand for connecting phalanx (finger) bones to the appropriate muscles. Flexor tendons, which are connected to the volar or palm side of the fingers, lend the ability to curl the fingers towards the palm. Extensor tendons, which are connected to the dorsal or backside of the fingers, return the curled fingers back into a straight position. Sheaths and retinacula restrain most tendons in the hand to some extent and keep them close to the skeletal plane so that they maintain a relatively constant moment arm rather than bowstringing across the joints. The pulley system of the flexor tendon sheath in the finger is the most highly developed of these restraints. The flexor tendon sheath pulley system permits the flexor tendons to maintain a relatively constant moment arm and helps minimize stress risers between tendon and sheath. This system serves three important functions. First, it allows smooth tendon gliding or lubrication; second, the retinacular reinforcing pulleys maintain the flexor tendons close to the surface of the finger bones, preventing bowstringing; and third, it provides an enclosed synovial fluid environment for tendon nutrition and lubrication. As the finger moves, each tendon slides a certain distance, which defines the “excursion of the tendon”. Excursion takes place simultaneously in the flexor and extensor tendons during joint motion. The tendons of the agonist, or contracting muscle, displace in one direction. The tendons of the antagonist or resisting muscles displace in the opposite direction to accommodate the motion.
Today, the most common methods of repairing torn, severed or otherwise damaged tendons involve approximating the severed ends of the tendons and suturing one side of the tendon to the other thereby returning the tendon to its natural position. A popular suture technique is the so-called Kessler technique and slight modifications thereof. Some of the other techniques include the Becker, Savage, lateral trap, double loop locking suture, four-strand interlock and variations of the Halsted technique. Other methods place prosthetic material either within or around the tendon. Polyester strips and sleeves along with polyester mesh have been used to reinforce the suture/tendon interface to provide a stronger repair.
Since most suture-based tendon repairs reach their tensile limit at about 6 lbs., surgeons must balance the desire to have full and immediate active motion to prevent adhesions against the need for immobilization to prevent rupture of the repair. Earlier loading of a repaired tendon promotes a more rapid increase in repair strength. For a tendon to properly rejoin, the opposed tendon ends do not have to touch but they do need to be approximated within 1-2 mm of each other to properly reattach. Tendons will heal at a rate that is proportional to the load being applied during physical therapy.
Similar problems and issues are encountered when attaching tendons or ligaments to bone. That is, simply suturing the tendon or ligament to a bone anchor or using external tendon anchor members may not provide the necessary strength of repair. As further discussed above, these techniques also promote adhesion formation.
U.S. Pat. No. 6,984,241 discloses examples of generally helical type coil anchors used for soft tissue repair, such as tendon or ligament repair. This includes tendon-to-tendon repair, ligament-to-ligament repair, and tendon/ligament-to-bone repair, for example. Various systems are disclosed in the '241 patent including one or more anchors and one or more flexible tensile members, for example, to effect the repair of a tendon or ligament, including soft tissue-to-bone techniques. The use of a generally helical coil was found to allow the soft tissue fibers to be held within the coil under compression applied in a direction generally perpendicular to the long axes of the fibers. This firmly couples the coil to the soft tissue, such as the tendon or ligament. This then allowed significant tensile force to be applied to the tendon or ligament during the repair and rehabilitation processes. For example, a flexible tensile member such as a suture could then be used to pull the tendon or ligament to a desired repair position, such as toward an opposing severed tendon or ligament end, or toward a bone.
Despite the significant improvements set forth in the '241 patent, there remains a need for improvements that allow for even greater repair strength and smaller anchor sizes, for example. Anchor systems that allow for larger tensile loads to be applied at the repair site will, for example, allow the patient to have even more active physical therapy after surgery and faster, more successful recovery. Smaller anchor sizes that still provide superior repair strength would, for example, provide a less obtrusive anchor system at the repair site and give the surgeon greater flexibility in making the repair and the ability to more easily repair smaller tendons or ligaments.